There are a variety of known ways to print hard copy. To name a few, the traditional techniques include rotogravure printing and offset lithography. Both of these printing methods require a plate which bears an image of the original document or picture to be copied and usually the plate is loaded onto a plate cylinder of a rotary press so that copies can be made efficiently. In the case of gravure printing, the plate cylinder is inked and the inked image thereon is impressed directly onto the paper or other copying medium. In the case of lithography, the image is present on a plate or mat as hydrophyllic and hydrophobic surface areas. Water tends to adhere to the water-receptive or hydrophyllic areas of the plate creating a thin film of water there which does not accept ink. The ink adheres to the hydrophobic areas of the plate. Those inked areas, usually corresponding to the printed areas of the original document (direct printing), are transferred to a relatively soft blanket cylinder and that, in turn, applies the image to the paper or other copying medium brought into contact with the surface of the blanket cylinder by an impression cylinder.
While certain aspects of the present invention are applicable to both kinds of printing and the approach can be applied to any number of colors including one as will be pointed out in more detail later, we will describe the invention in the context of a sheet-fed four-color offset press.
The plates for an offset press are usually produced photographically. In a typical negative-working subtractive process, the original document is photographed to produce a photographic negative. The negative is placed on an aluminum plate having a water-receptive oxide surface that is coated with a photopolymer. Upon being exposed to light through the negative, the areas of the coating that received light (corresponding to the dark or printed areas of the original) cures to a durable oleophyllic or ink-receptive state. The plate is then subjected to a developing process which removes the noncured areas of the coating that did not receive light (corresponding to the light or background areas of the original) and these non-cured areas become hydrophyllic (water loving). The resultant plate now carries a positive or direct image of the original document.
If a press is to print in more than one color, a separate printing plate corresponding to each color is required, each of which is usually made photographically as just described. In addition to preparing the appropriate plates for the different colors, the plates must be mounted properly on the plate cylinders in the press and the positions of the cylinders coordinated so that the color components printed by the different cylinders will be in register on the printed copies.
In most conventional presses, the printing stations required to print the different colors are arranged in a straight line or flatbed approach. Each such station contains all of the elements required to print a single color, including an impression cylinder, a blanket cylinder, a plate cylinder and the necessary ink and water systems for applying ink and water to the plate cylinder. The equaldiameter plate and blanket cylinders at each station are geared to the impression cylinder there and the latter is geared to the impression cylinders in the other stations so that all of the press cylinders rotate in unison to maintain registration of the different color components of each copy.
To make a copy on that type of press, a sheet of paper is fed to the first print station where its leading edge is gripped and the sheet wrapped around the impression cylinder at that station. The press then operates to print onto the sheet, say, the cyan color component of the original document being copied, after which that sheet is discharged to the second printing station of the press. At station No. 2, the leading edge of the sheet is picked up by a second gripper and wrapped around the impression cylinder of that station. The press then operates to print a second, e.g. the yellow, color component of the original document onto the paper sheet, after which the sheet is discharged to the third printing station which grabs the sheet and prints the third color component, i.e. magenta, onto the sheet. In four-color printing, the sheet passes through a fourth station which prints a black image onto the sheet. Thus, successive paper sheets are fed into the press, are printed on at the various print stations thereof, and then exit the press carrying a three or four-color image of the original document or picture.
A conventional press such as the one just described has several drawbacks. First of all, since it consists essentially of three or four single color presses arranged one after the other, it occupies a considerable amount of floor space. A present day four-color press of this type can be as long as 20 feet. Secondly, the sheet has to be picked up and wrapped around the impression cylinder at each print station of the press. Thus, in a four-color press, four separate operations are required to position the sheet for printing. This means that each printing station must have its own paper feeding and handling mechanisms. Not only does this increase the cost of the press, it also introduces print registration errors into the printed copies.
Normally in a press, misregistrations are corrected for by manually or automatically adjusting the relative positions of the plate cylinders at the various print stations in a proper rotational, axial, and skew-orientation phase. It has been proposed that by imaging the plates "on press" the time required to correct for misregistration will be substantially decreased. The imaging of the plates can be controlled by incoming image signals representing the original document to be copied or reproduced in high volume. Indeed, it has been proposed to image an offset plate on the press using an ink jetter. The ink jetter is controlled so as to deposit on the plate surface a thermoplastic imageforming resin or material which has a desired affinity for the printing ink being used to print the copies.
While that proposed system may be satisfactory for some applications, it is not always possible to provide thermoplastic image-forming material that is suitable for jetting and also has the desired affinity (phyllic or phobic) for the inks commonly used to make lithographic copies. Further, ink jet printers are generally unable to produce small enough ink dots to allow the production of smooth, continuous tones on the printed copies, i.e. the resolution is not high enough.
In any event, such manual, automatic or electronic registration correction procedures are not totally satisfactory for a sheet fed press because the registration errors due to the multiple grippings of each sheet are random errors that cannot be corrected completely by onetime adjustments of the plate cylinders or of the images thereon. Nor are such procedures effective to correct for misregistration due to random gearing errors caused by variations in the tooth profiles of the meshing gears that drive the various cylinders of the press. These tooth profile variations arise in the process of cutting the gears and they are more noticeable in large diameter gears.
Since such random errors are not normally correctable, press manufacturers have had to resort to minimizing the problem by using very accurate paper feeding mechanisms and precision gearing. Such precision parts are quite expensive and materially increase the overall cost of the press. Also, as alluded to above, the misregistration problem is not completely eliminated and can still manifest itself in a press intended to print high quality, high resolution copies, which is the type of press we are primarily concerned with here.
Thus, although considerable effort has been devoted to improving different aspects of printing, including lithographic printing, there still does not exist a compact, relatively low cost printing apparatus or press whose printing plates or cylinders can be formed right on the press using incoming digital data representing original documents or pictures to enable the printing in long or short runs of high quality continuous tone color reproductions or copies. It would, therefore, be highly desirable if such apparatus could be made available particularly as a relatively compact sheet fed press and at a cost affordable to printers and other businessmen who want to do high quality printing and publishing in-house.